1. Field of Invention
The present invention relates to a bifunctional compound with a monosaccharide and a N2S2 ligand, and preparation and use thereof, and more particularly to a bifunctional compound with aminohexyl N-acetylgalactosamine (ah-GalNAc).
2. Related Art
In studies on non-invasive detection of liver fibrosis, imaging in nuclear medicine with radio-labeled glycoprotein reflecting liver function is generally performed with liver asialoglycoprotein receptor (ASGPR), which is applied in test of residual liver function in a human.
ASGPR is a phagocytic receptor merely existing on surface of mammal liver cells, and can specifically identify sugar chains with a terminal galactosamine (Gal) or N-acetylgalactosamine (GalNAc). Hashida et al (Hashida M, Nishidawa M, Tadakura Y., J Controlled Release, 36(1):99, (1995)) concluded through comparison with experiments that, in view of reflection of liver function, SPECT imaging of liver ASGPR is significantly superior to other imaging means. ASGPR is high affinity to liver, and can be quickly absorbed by the liver. A number of ASGPR changes when lesions occur to the liver, and thus, a novel contrast medium for liver cell fibrosis in nuclear medicine can be developed based on this.
Currently, in studies on ASGPR contrast medium, a bifunctional compound is generally used, and Tc-99m is confirmed to be stably chelated in coordination bonding with a series of bifunctional ligands containing N2S2, to form a complex. A N2S2 ligand is bonded with Tc-99m in a five coordination manner, in which coordination binding occurs between Tc-99m and N2S2, thus forming a stable pyramid configuration. Types and structures of common N2S2 bifunctional chelating agents are as follows (Chryssou K, Inorganica Chimica Acta Vol. 268,169 (1998)).

It was suggested to use a radiopharmaceutical Tc-99m-DTPA-galactosyl-albumin (a liver asialoglycoprotein) as a contrast medium in nuclear medicine of liver asialoprotein receptor in imaging of liver function (Toyama H, Suzuki K, Naito A et al, Ann Nucl Med, 13, 155-160, (1999)). Although a general test agent can rapidly evaluate the liver function in vitro, how much function of the liver is left cannot be known. However, with Tc-99m-DTPA-galactosyl-albumin as a radiopharmaceutical, not only how many liver cells are alive, but also which regions still function can be known. Clinical significance of a contrast medium of liver asialoprotein in nuclear medicine lies in that, as transient hypoxia of liver cells is frequently caused after liver transplantation, it is necessary to determine how many liver cells are still alive with galactosyl-albumin-DTPA-Tc-99m after operation, so as to determine the success of the liver transplantation.
Presently, many researches on ligands that specifically bind to the liver cells are reported, and the ligands are widely used. For example, a research (Khorev O, Stokmaier D, Schwardt O, Cutting B, Ernst B. Trivalent, Bioorg Med Chem; 16:5216-5231. (2008)) indicates that synthesized ligands or peptides containing trivalent β-linked Gal or GalNAc branched saccharide structure have high affinity and specificity to ASGPR, and can be selectively endocytosed by HepG2 liver cell lines. Another research also verifies that endocytosis mediated by binding of ligands with a Gal/GalNAc terminal to ASGPR is regulated by calcium ion (Kim S H, Goto M, Akaike T. J Biol Chem; 276:35312-35319. (2001)).
As Gal/GalNAc has specificity to hepatic lectin, a radiopharmaceutical where a radioactive isotope is bound to a Gal/GalNAc glycoprotein can be successfully positioned to the liver cells and be endocytosed by the liver cells, thus achieving a purpose of function imaging or radiotherapy. Presently, a ah-GalNAc saccharide group has been prepared (Lee, R. T.; Wong, T. C. and Lee, Y. C., J. Carbohydrate Chem., 5, 343-357 (1986)), with a preparation process as follows. However, the preparation of the ah-GalNAc saccharide group has disadvantages below.

1. Synthesis methods in literatures are mainly directed to biochemistry field, and are not convenient for common organic synthesis laboratories.
2. There are many amino protecting groups, and thus selection of the amino protecting group is dependent on use of a protected compound. During synthesis of ah-GalNAc in literatures, an amino group in 6-amino hexanol is protected with trifluoacetyl, and the protecting group needs to be removed subsequently, which is inconvenient for treatment.
3. In the methods in literatures, LC columns made of expensive resins such as Sephadex LH-20 are requred for separation and purification. However, Sephadex LH-20 is very expensive and cannot be afforded by common laboratories, so the cost is high in terms of economic.
Furthermore, no attmpt is made to inject ASGPR contrast medium or other novel contrast agents with high specificity for liver into a human body currently. Combination of ASGPR contrast medium with the PET-CT technology, in cooperation with a receptor binding bifunctional compound may be one of important directions in development of liver function display using imaging methods. However, there is no contrast medium in nuclear medicine for a liver cell receptor formed by combining galactosamine (Gal) and N-acetylgalactosamine (GalNAc) with N2S2 ligand.